NullPointerException of JDBC Template in DAO Constructor - spring

Question

I am new to Spring Boot and was trying to implement the below code for Spring Boot JDBC. I am getting a NullPointerException in the this.jdbcTemplate in the UserDao constructor while running the application. I think the error is because the UserDao constructor is being called before the instantiation of JdbcTemplate. Is that correct?
I haven't come across this error until now where the #Autowired cannot resolve the dependencies. Can someone please elaborate upon the reason for the error?
SpringBootDatabaseApplication.java
package com.mrityu.springbootdatabase;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.EnableAutoConfiguration;
import org.springframework.boot.autoconfigure.SpringBootApplication;
#SpringBootApplication
public class SpringBootDatabaseApplication {
#Autowired
private UserDao userDao;
public static void main(String[] args) {
SpringApplication.run(SpringBootDatabaseApplication.class, args);
}
}
UserDao.java
package com.mrityu.springbootdatabase;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.stereotype.Repository;
#Repository
public class UserDao {
#Autowired
private JdbcTemplate jdbcTemplate;
public UserDao() {
String query = "create table if not exists User(id int primary key, name varchar(200))";
int update = this.jdbcTemplate.update(query);
System.out.println("Constructor Called: " + update);
}
}

Answer 1

I didn't try to run the code, but you're trying to use the uninitialized jdbcTemplate field in your constructor.
Constructor base injection should do the trick
public UserDao(JdbcTemplate jdbcTemplate) {...}
Otherwise, you can still use field injection and move your logic into a method.
public void yourMethod() {
String query = "create table if not exists User(id int primary key, name varchar(200))";
int update = this.jdbcTemplate.update(query);
System.out.println("Method Called: " + update);
}
In your case, it's about how java initializes classes, not spring boot auto wiring.
You can check this answer for the dependency injection precedence.